Variable-gain amplifier circuit

ABSTRACT

There is disclosed a variable-gain amplifier circuit comprising an adjustable impedance circuit part including a bipolar transistor having a base connected to a control terminal for receiving a controlled bias voltage, a collector connected to a positive power supply voltage terminal, and an emitter connected to ground through a diode in such a manner that an anode of the diode is connected to the emitter of the transistor and a cathode of the diode is connected to the ground. The variable-gain amplifier circuit further comprises a variable-gain amplifying circuit part having an amplifying circuit having an input coupled through an DC blocking capacitor to an input terminal for receiving an input signal and an output connected to an output terminal. The emitter of the bipolar transistor is connected to a node between the input terminal and the DC blocking capacitor. By controlling the bias voltage applied to the base of the transistor, it is possible to adjust the gain of the whole circuit by action of the adjustable impedance circuit part, and at the same time, it is possible to allow the amplifying circuit having a high S/N ratio, to operate under a low power supply voltage with a low noise factor.

BACKGROUND OF THE INVENTION

The present invention relates to a gain control circuit, which can beused in various parts of a low-noise amplifier used in atransmitter/receiver of mobile radio communication equipment, and whichcan operate as a variable gain circuit for compensating for theattenuation amount of a radio wave.

In general, in the field of radio communication, the attenuation amountof a radio wave changes with a communication distance. In mobile radiocommunication equipment such as mobile telephone, since thecommunication distance greatly changes, it has been required with avariable-gain amplifier circuit in order to compensate for theattenuation amount of the radio wave dependently upon the communicationdistance.

Referring to FIG. 10, there is shown a block diagram illustrating afundamental construction of a transmitter/receiver section in a priorart mobile radio communication equipment. The shown transmitter/receiversection includes a receiver circuit part connected between one selectionterminal of a switch 102 connected to an antenna 101 and one terminal ofa base band processing part 113, the receiver circuit part including alow-noise amplifier 103, a band-pass filter (BP) 104, a mixer 108, anintermediate frequency (IF) amplifier 105, a band pass filter (BP) 106and a mixer 108, which are cascade-connected in the named order. Theshown transmitter/receiver section includes a transmitter circuit partconnected between another terminal of the base band processing part 113and the other selection terminal of the switch 102, the transmittercircuit part including a mixer 109, a band pass filter (BP) 112, adriver amplifier 111 and a power amplifier 110, which arecascade-connected in the named order.

In the above mentioned arrangement, the low-noise amplifier 103 and theintermediate frequency amplifier 105 in the receiver circuit part andthe driver amplifier 111 in the transmitter circuit part can beconstituted of the above mentioned variable-gain amplifier circuit.

Referring to FIG. 11, there is shown a circuit diagram illustrating abasic construction of the variable-gain amplifier circuit which can beused at various parts in the transmitter/receiver section of the mobileradio communication equipment. The shown variable-gain amplifier circuitis constructed to have a function similar to that of the variable-gainamplifier circuit disclosed in Japanese Patent Application No. Heisei10-260734 published as JP-A-2000-091861. This variable-gain amplifiercircuit includes an adjustable impedance circuit part including an NPNbipolar transistor 8 having a base connected to a control terminal 5 forreceiving a controlled bias voltage, a collector connected to a powersupply terminal 11 for receiving a positive fixed voltage, and anemitter connected through a resistor 6 to ground. The variable-gainamplifier circuit also includes a variable-gain amplifying circuit parthaving an amplifying circuit 1 having an input connected through acapacitor 9 to an input terminal 3 for receiving an input signal and anoutput connected to an output terminal 4, a node between the inputterminal 3 and the capacitor 9 being connected to a node between theemitter of the transistor 8 and the resistor 6.

With this arrangement, the power supply terminal 11 connected to thecollector of the transistor 8 is connected through a load (not shown) toa constant positive voltage power supply, and the control terminal 5connected to the base of the transistor 8 is connected to a base biascircuit (not shown) which is formed of for example two bias resistors.In addition, as mentioned above, the emitter of the transistor 8 isconnected through the resistor 6 to the ground. Thus, the adjustableimpedance circuit part is constituted of the transistor 8 itself and theresistor 6. Since a constant positive voltage is applied to thecollector of the transistor through the load from the power supply, thecapacitor 9 functions as a DC blocking capacitor against the adjustableimpedance circuit part when an amplified signal is outputted from theoutput terminal 4 of the amplifying circuit 1 in the variable-gainamplifier circuit.

In this variable-gain amplifier circuit, the shunt amount to the ground,of an input signal supplied to the input terminal 3, is controlled bythe adjustable impedance circuit part, so that the amplification gain ofthe amplifying circuit 1 in the variable-gain amplifying circuit partcan be varied. More specifically, since the control terminal 5 connectedto the base of the transistor 8 is grounded in an AC mode, if the biasvoltage applied to the control terminal 5 connected to the base of thetransistor 8 is controlled, an impedance between the emitter of thetransistor 8 and the ground (impedance of the transistor 8 and theresistor 6) changes, with the result that the shunt amount of the inputsignal supplied to the input terminal 3 is controlled, and therefore,the amplification gain of the amplifying circuit 1 changes.

As other known technique concerning the gain control circuit and thevariable-gain amplifier circuit, a variable-gain amplifier circuit shownin JP-A-6-120756 and a gain control circuit shown in JP-A-2000-151311are exemplified.

In the above mentioned gain control circuit (variable-gain amplifiercircuit), if the resistance value of the resistor 6 connected to theemitter of the transistor in the adjustable impedance circuit part issmall, the input signal is bypassed through the resistor directly to theground, so that a SIN ratio (ratio of signal power to noise power) inthe amplifying circuit lowers, with the result that a noise factor(called an “NF”) in a maximum gain condition becomes large. To thecontrary, if the resistance value of the resistor 6 connected to theemitter of the transistor is large, the power supply voltage requiredfor operating the transistor becomes large. Accordingly, if theresistance value of the resistor 6 is not set at an appropriate value, astable operation cannot be obtained.

Specifically, in the gain control circuit shown in FIG. 11, assumingthat the power supply voltage is 3.0V, a collector current of thetransistor 8 in operation is 5.0 mA, a collector-emitter voltage drop ofthe transistor 8 is 1.0V, a maximum value Re_(max) of the resistor 6 forallowing the transistor 8 to operate is expressed as the followingequation (1): $\begin{matrix}{{Re}_{\max} = {\frac{3.0 - 1.0}{5.0 \times 10^{- 3}} = {400\quad \Omega}}} & (1)\end{matrix}$

FIG. 12 illustrates a relation of the noise factor NF (dB) to theresistance value Re (ohm) of the resistor 6 in the maximum gaincondition of this gain control circuit (resistance dependency of NF).Here, the characteristics of {ONLY RESISTOR} is referred to. Forexample, if it is desired to suppress the noise factor NF to not greaterthan 1.2 dB, the resistor is required to have the resistance of not lessthan 1.2 KΩ, which is apparently larger than 400 Ω as mentioned above.This means that the transistor 8 cannot operate under the power supplyvoltage of 3.0V.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a gaincontrol circuit which has overcome the above mentioned problems of theprior art.

Another object of the present invention is to provide a gain controlcircuit allowing a variable-gain amplifier circuit to operate under alow power supply voltage with a lowered noise factor in the maximum gaincondition.

A further object of the present invention is to provide a variable-gainamplifier circuit incorporating such a gain control circuit therein.

The above and other objects of the present invention are achieved inaccordance with the present invention by a variable-gain amplifiercircuit comprising;

an adjustable impedance circuit part including a bipolar transistorhaving a base connected to a control terminal for receiving a controlledbias voltage, one of a collector and an emitter of the bipolartransistor being connected to a first power supply terminal, and theother of the collector and the emitter of the bipolar transistor beingconnected to a second power supply terminal through a diode located in aforward direction to the second power supply terminal; and

a variable-gain amplifying circuit part having an amplifying circuithaving an input coupled to an input terminal for receiving an inputsignal and an output connected to an output terminal,

the other of the collector and the emitter of the bipolar transistorconnected to the second power supply terminal through the diode beingconnected through an DC blocking capacitor to the input of theamplifying circuit.

Preferably, the diode is connected in series with a resistor between theother of the collector and the emitter of the bipolar transistor and thesecond power supply terminal.

In a first embodiment of the variable-gain amplifier circuit, the firstpower supply terminal is a positive power supply voltage terminal andthe second power supply terminal is connected to ground, the collectorof the bipolar transistor being connected to the positive power supplyvoltage terminal, and the emitter of the bipolar transistor beingconnected to the ground through the diode connected in such a mannerthat an anode of the diode is connected to the emitter of the transistorand a cathode of the diode is connected to the ground. Furthermore, theinput of the amplifying circuit is connected through the DC blockingcapacitor to the input terminal, and the emitter of the bipolartransistor is connected to a node between the DC blocking capacitor andthe input terminal.

In this first embodiment, a resistor can be connected in series with thediode between the emitter of the bipolar transistor and the ground.

In a second embodiment of the variable-gain amplifier circuit, the firstpower supply terminal is connected to ground and the second power supplyterminal is a positive power supply voltage terminal, the emitter of thebipolar transistor being connected to the ground, and the collector ofthe bipolar transistor being connected to the positive power supplyvoltage terminal through the diode connected in such a manner that ananode of the diode is connected to the positive power supply voltageterminal and a cathode of the diode is connected to the collector of thebipolar transistor. Furthermore, the input of the amplifying circuit isconnected through the DC blocking capacitor to the input terminal, andthe collector of the bipolar transistor is connected to a node betweenthe DC blocking capacitor and the input terminal.

In this second embodiment, a resistor can be connected in series withthe diode between the collector of the bipolar transistor and thepositive power supply voltage terminal.

In a third embodiment of the variable-gain amplifier circuit, the firstpower supply terminal is a positive power supply voltage terminal andthe second power supply terminal is ground, the collector of the bipolartransistor being connected to the positive power supply voltageterminal, and the emitter of the bipolar transistor being connected tothe ground through the diode connected in such a manner that an anode ofthe diode is connected to the emitter of the transistor and a cathode ofthe diode is connected to the ground. Furthermore, the input of theamplifying circuit is connected directly to the input terminal, and theemitter of the bipolar transistor is connected through the DC blockingcapacitor to the input of the amplifying circuit.

In this third embodiment, a resistor can be connected in series with thediode between the emitter of the bipolar transistor and the ground.

In a fourth embodiment of the variable-gain amplifier circuit, the firstpower supply terminal is connected to ground and the second power supplyterminal is a positive power supply voltage terminal, the emitter of thebipolar transistor being connected to the ground, and the collector ofthe bipolar transistor being connected to the positive power supplyvoltage terminal through the diode connected in such a manner that ananode of the diode is connected to the positive power supply voltageterminal and a cathode of the diode is connected to the collector of thebipolar transistor. Furthermore, the input of the amplifying circuit isconnected directly to the input terminal, and the collector of thebipolar transistor is connected through the DC blocking capacitor to theinput of the amplifying circuit.

In this fourth embodiment, a resistor can be connected in series withthe diode between the collector of the bipolar transistor and thepositive power supply voltage terminal.

The above and other objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the variable-gain amplifier circuitincorporating therein a first embodiment of the gain control circuit inaccordance with the present invention;

FIG. 2 is an AC equivalent circuit of an adjustable impedance circuitpart in the gain control circuit shown in FIG. 1;

FIG. 3 is a circuit diagram of a modification of the first embodimentshown in FIG. 1;

FIG. 4 is a circuit diagram of the variable-gain amplifier circuitincorporating therein a second embodiment of the gain control circuit inaccordance with the present invention;

FIG. 5 is a circuit diagram of a modification of the second embodimentshown in FIG. 4;

FIG. 6 is a circuit diagram of the variable-gain amplifier circuitincorporating therein a third embodiment of the gain control circuit inaccordance with the present invention;

FIG. 7 is a circuit diagram of a modification of the third embodimentshown in FIG. 6;

FIG. 8 is a circuit diagram of the variable-gain amplifier circuitincorporating therein a fourth embodiment of the gain control circuit inaccordance with the present invention;

FIG. 9 is a circuit diagram of a modification of the fourth embodimentshown in FIG. 8;

FIG. 10 is a block diagram illustrating a fundamental construction of atransmitter/receiver section in a prior art mobile radio communicationequipment;

FIG. 11 is a circuit diagram illustrating a basic construction of thevariable-gain amplifier circuit which can be used at various parts inthe transmitter/receiver section of the mobile radio communicationequipment shown in FIG. 10; and

FIG. 12 is a graph illustrating a relation of the noise factor NF (dB)to the resistance value Re (ohm) of the resistor in the maximum gaincondition of the gain control circuits shown in FIGS. 3 and 11.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a circuit diagram of thevariable-gain amplifier circuit incorporating therein a first embodimentof the gain control circuit in accordance with the present invention. InFIG. 1 and the succeeding figures, elements corresponding to those shownin FIG. 11 are given the same Reference Numbers.

The shown circuit includes an adjustable impedance circuit partincluding an NPN bipolar transistor 8 having a base connected to acontrol terminal 5 for receiving a controlled bias voltage, a collectorconnected to a power supply terminal 11 for receiving a positive fixedvoltage, and an emitter connected to ground through a diode 2 connectedin a forward direction to the ground. The shown circuit also includes avariable-gain amplifying circuit part having an amplifying circuit 1having an input connected through a capacitor 9 to an input terminal 3for receiving an input signal and an output connected to an outputterminal 4. A node between the input terminal 3 and the capacitor 9 isconnected to a node between the emitter of the transistor 8 and thediode 2. Since the diode 2 is connected in the forward direction to theground, an anode of the diode 2 is connected to the emitter of thetransistor 8, and a cathode of the diode 2 is connected to the ground.

As seen from comparison between FIG. 1 and FIG. 11, the shown circuit isfeatured in that the diode 2 is provided in place of the resistor 6 inthe adjustable impedance circuit part. Therefore, input and outputterminals of the shown circuit are connected to peripheral circuits anddevices (not shown), similarly to the circuit shown in FIG. 11.

With his arrangement, the power supply terminal 11 connected to thecollector of the transistor 8 is connected through a load (not shown) toa constant positive voltage power supply, and the control terminal 5connected to the base of the transistor 8 is connected to a base biascircuit (not shown) which is formed of for example two bias resistors.In addition, as mentioned above, the emitter of the transistor 8 isconnected through the diode 2 to the ground. Thus, the adjustableimpedance circuit part is constituted of the transistor 8 itself and thediode 2. Since a constant positive voltage is applied to the collectorof the transistor through the load from the power supply, the capacitor9 functions as a DC blocking capacitor against the adjustable impedancecircuit part when an amplified signal is outputted from the outputterminal 4 of the amplifying circuit 1 in the variable-gain amplifiercircuit.

Also in this variable-gain amplifier circuit, the shunt amount to theground, of an input signal supplied to the input terminal 3, iscontrolled by the adjustable impedance circuit part, so that theamplification gain of the amplifying circuit 1 in the variable-gainamplifying circuit part can be varied. More specifically, since thecontrol terminal 5 connected to the base of the transistor 8 is groundedin an AC mode, if the bias voltage applied to the control terminal 5connected to the base of the transistor 8 is controlled, an impedancebetween the emitter of the transistor 8 and the ground changes becausethe impedance changes in accordance with the amount of currents flowingthrough the transistor 8 and through the diode 2, with the result thatthe shunt amount of the input signal supplied to the input terminal 3 iscontrolled, and therefore, the amplification gain of the amplifyingcircuit 1 changes.

Now, an operation in this gain control circuit will be explained.

A relation expressed in the following equation (2) holds among animpedance Z_(D), a differential conductance g_(D) and a capacitance Cjof the diode 2. $\begin{matrix}{\frac{1}{Z_{D}} = {g_{D} + {j\quad \omega \quad C_{j}}}} & (2)\end{matrix}$

Here, when a diffusion capacitance is sufficiently smaller than ajunction capacitance, the differential conductance g_(D) and thecapacitance Cj can be expressed as the following equations (3) and (4),respectively. $\begin{matrix}\begin{matrix}{g_{D} = {{\frac{q}{K\quad T} \cdot I_{F}} = {g_{O}\left( {e^{a_{O} \cdot V_{D}} - 1} \right)}}} & \left( {V_{D} \geqq 0} \right)\end{matrix} & (3) \\\begin{matrix}{{Cj} = \left( \frac{C}{\varphi_{D} - V_{D}} \right)^{\frac{1}{n}}} & \left( {V_{D} \geqq \varphi_{D}} \right)\end{matrix} & (4)\end{matrix}$

where

q is electric charge amount of electron

k is Boltzmann's constant

T is absolute temperature

I_(F) is a forward DC current of the diode 2

g₀ and a₀ are constant

φ_(D) is built-in potential

n is constant which ordinarily takes the value of 2 to 3

C is coefficient of the junction capacitance and becomes very small byreducing the area of the diode 2

V_(D) is forward voltage applied to the diode 2

Incidentally, if the forward voltage V_(D) applied to the diode 2 isnegative, the differential conductance g_(D) becomes zero, and if theforward voltage V_(D) applied to the diode 2 is positive, thedifferential conductance g_(D) becomes large in proportion to theforward DC current I_(F). In addition, the capacitance Cj is acapacitance created by a depletion layer generated in a p-n junction,and becomes infinite when the forward voltage V_(D) reaches the built-inpotential φ_(D).

Referring to FIG. 2, there is shown an AC equivalent circuit of theadjustable impedance circuit part in the gain control circuit shown inFIG. 1. This AC equivalent circuit includes the transistor 8, the diode2 and a current source. The impedance Z_(D) of the diode 2 between abase side and an emitter side in the transistor 8 corresponds to theimpedance Z_(D) expressed in the above equation (2). Beside, a parasiteresistance in the emitter side of the transistor 8 is expressed byr_(e), and a parasite resistance in the base side of the transistor 8 isexpressed by r_(b). A transport factor in the current source isexpressed as α. In this case, the impedance Z_(i) viewed from theemitter of the transistor 8 is expressed by the following equation (5):$\begin{matrix}{Z_{i} = \frac{v_{1}}{i_{e}}} & (5)\end{matrix}$

where V₁ is expressed by the following equation (5): $\begin{matrix}{v_{1} = {\frac{r_{e} + Z_{D}}{i_{e}} + r_{b} + i_{b}}} & (6)\end{matrix}$

In addition, the following relation (7) holds in the transistor 8.

i _(b) =i _(e) −i _(c)=(1−α)·i _(e)  (7)

Accordingly, the impedance Z_(i) is expressed as follows:

Z _(i) =r _(e) +Z _(D)+(1−α)·r _(b)  (8)

In conclusion, the impedance Z_(i) viewed from the emitter of thetransistor 8 assumes the following maximum value when a voltage betweenthe base side and the emitter side is 0 (zero).$r_{e} + \left( \frac{\varphi_{D}}{C} \right)^{\frac{1}{n}} + {\left( {1 - \alpha} \right) \cdot r_{b}}$

As mentioned above, the maximum value is very large, but the larger thevoltage between the base side and the emitter side is, the impedanceapproaches 0 (zero).

Applying this to the gain control circuit shown in FIG. 1, if thecontrolled bias voltage applied to the control terminal 5 is increased,the voltage between the base side and the emitter side in the transistor8 becomes large, so that the impedance Z_(i) of the transistor 8 viewedfrom the emitter of the transistor 8 becomes small, with the result thatthe input signal applied to the input terminal 3 is shunted. Inaddition, since the collector current of the transistor 8 flows into thediode 2, the impedance Z_(D) of the diode 2 lowers in accordance withthe equation (2), so that the input signal is also shunted into thediode 2. As a result, the gain of the whole circuit lowers.

To the contrary, if the bias voltage applied to the control terminal 5is decreased, both the impedance Z_(i) of the transistor 8 viewed fromthe emitter and the impedance Z_(D) of the diode 2 become large, withthe result that the proportion of the input signal actually applied tothe amplifying circuit 1 is increased, so that the gain of the wholecircuit elevates. Furthermore, when the bias voltage applied to thecontrol terminal 5 is made smaller than a voltage obtained by adding anon-voltage of the diode 2 to an on-voltage between the base and theemitter of the transistor 8, the respective impedances become extremelylarge, and therefore, all of the input signal is supplied to theamplifying circuit 1. Accordingly, the gain increases. At this time, theS/N ratio of the whole circuit becomes large so that the noise factor NFbecomes small.

Referring to FIG. 3, there is shown a circuit diagram of a modificationof the first embodiment shown in FIG. 1. This modification is differentfrom the first embodiment shown in FIG. 1 only in that a resistor 6 isinserted between the cathode of the diode 2 and the ground.

In this circuit, assuming that the power supply voltage is 3.0V, acollector current of the transistor 8 in operation is 5.0 mA, acollector-emitter voltage drop of the transistor 8 is 1.0V, when thetransistor 8 is off, the diode 2 is also off, so that a very highimpedance is realized. Therefore, even if the value of the resistor 6 issmall, the value of the noise factor NF becomes sufficiently small, asshown by the characteristics of {RESISTOR+DIODE} in FIG. 12. Inaddition, assuming that the voltage drop of the diode 2 in the oncondition is 1.4V and the collector-emitter voltage drop of thetransistor 8 is 1.0V, if the value of the resistor 6 is made small, thecircuit can operate under the power supply voltage of not less than2.4V.

Incidentally, it would be apparent to persons skilled in the art thatsimilar effect and advantage can be obtained even if the diode 2 and theresistor 6 is exchanged in position so that the resistor 6 is connectedbetween the emitter of the transistor 8 and the anode of the diode 2.

The fundamental operation of the above mentioned gain control circuitcan be obtained even if the place of the diode 2 and the resistor 6 inrelation to the transistor 8 and the position of the DC blockingcapacitor 9 are changed as follows:

Referring to FIG. 4, there is shown a circuit diagram of thevariable-gain amplifier circuit incorporating therein a secondembodiment of the gain control circuit in accordance with the presentinvention.

The circuit shown in FIG. 4 includes an adjustable impedance circuitpart including a bipolar transistor 8 having a base connected to acontrol terminal 5 for receiving a controlled bias voltage, a collectorconnected to a positive power supply terminal 11 through a diode 2connected in a forward direction to the positive power supply terminal11, and an emitter connected directly to ground. The power supplyterminal 11 is connected for receiving a fixed positive power supplyvoltage. The shown circuit also includes a variable-gain amplifyingcircuit part having an amplifying circuit 1 having an input connectedthrough a capacitor 9 to an input terminal 3 for receiving an inputsignal and an output connected to an output terminal 4. A node betweenthe input terminal 3 and the capacitor 9 is connected to a node betweenthe collector of the transistor 8 and the diode 2. Since the diode 2 isconnected in the forward direction to the positive power supply terminal11, an anode of the diode 2 is connected to the positive power supplyterminal 11, and a cathode of the diode 2 is connected to the collectorof the transistor 8.

Referring to FIG. 5, there is shown a circuit diagram of a modificationof the second embodiment shown in FIG. 4. This modification is differentfrom the second embodiment shown in FIG. 4 only in that a resistor 6 isadded between the anode of the diode 2 and the power supply terminal 11.In this connection, it would be apparent to persons skilled in the artthat the diode 2 and the resistor 6 can be exchanged in position so thatthe resistor 6 is connected between the collector of the transistor 8and the cathode of the diode 2.

Referring to FIG. 6, there is shown a circuit diagram of thevariable-gain amplifier circuit incorporating therein a third embodimentof the gain control circuit in accordance with the present invention.

The circuit shown in FIG. 6 includes an adjustable impedance circuitpart including a bipolar transistor 8 having a base connected to acontrol terminal 5 for receiving a controlled bias voltage, a collectorconnected to a power supply terminal 11 for receiving a fixed positivevoltage, and an emitter connected to ground through a diode 2 connectedin a forward direction to the ground. The shown circuit also includes avariable-gain amplifying circuit part having an amplifying circuit 1having an input connected directly to an input terminal 3 for receivingan input signal and an output connected to an output terminal 4. Theinput of the amplifying circuit 1 is connected through a capacitor 9 toa node between the emitter of the transistor 8 and the diode 2. Sincethe diode 2 is connected in the forward direction to the ground, ananode of the diode 2 is connected to the emitter of the transistor 8,and a cathode of the diode 2 is connected to the ground.

Referring to FIG. 7, there is shown a circuit diagram of a modificationof the third embodiment shown in FIG. 6. This modification is differentfrom the third embodiment shown in FIG. 6 only in that a resistor 6 isadded between the cathode of the diode 2 and the ground. In thisconnection, it would be apparent to persons skilled in the art that thediode 2 and the resistor 6 can be exchanged in position so that theresistor 6 is connected between the emitter of the transistor 8 and theanode of the diode 2.

Referring to FIG. 8, there is shown a circuit diagram of thevariable-gain amplifier circuit incorporating therein a fourthembodiment of the gain control circuit in accordance with the presentinvention.

The circuit shown in FIG. 8 includes an adjustable impedance circuitpart including a bipolar transistor 8 having a base connected to acontrol terminal 5 for receiving a controlled bias voltage, a collectorconnected to a positive power supply terminal 11 through a diode 2connected in a forward direction to the positive power supply terminal11, and an emitter connected directly to ground. The positive powersupply terminal 11 is connected for receiving a fixed positive powersupply voltage. The shown circuit also includes a variable-gainamplifying circuit part having an amplifying circuit 1 having an inputconnected directly to an input terminal 3 for receiving an input signaland an output connected to an output terminal 4. The input of theamplifying circuit 1 is connected through a capacitor 9 to a nodebetween the collector of the transistor 8 and the diode 2. Since thediode 2 is connected in the forward direction to the positive powersupply terminal 11, an anode of the diode 2 is connected to the positivepower supply terminal 11, and a cathode of the diode 2 is connected tothe collector of the transistor 8.

Referring to FIG. 9, there is shown a circuit diagram of a modificationof the fourth embodiment shown in FIG. 8. This modification is differentfrom the fourth embodiment shown in FIG. 8 only in that a resistor 6 isadded between the cathode of the diode 2 and the collector of thetransistor 8. In this connection, it would be apparent to personsskilled in the art that the diode 2 and the resistor 6 can be exchangedin position so that the resistor 6 is connected between the anode of thediode 2 and the positive power supply terminal 11.

Of the above mentioned embodiments, the gain control circuit shown inFIG. 4 and the gain control circuit shown in FIG. 8 operate similarly toeach other. In these gain control circuits, the cathode of the diode 2is connected to the collector of the transistor 8, and the anode of thediode 2 is connected to the positive power supply terminal 11 forreceiving a fixed positive voltage. The emitter of the transistor 8 isconnected directly to the ground. The control terminal 5 connected tothe base of the transistor 8 is connected to the ground in an AC mode.The collector of the transistor 8 is connected through the capacitor 9is connected to the input of the amplifying circuit 1. By controllingthe bias voltage at the control terminal 5 to change the current amountflowing through the diode 2, the impedance of the diode 2 changes, withthe result that the shunt amount of the input signal supplied to theinput terminal 3 is controlled, and therefore, the amplification gain ofthe amplifying circuit 1 changes.

More specifically, if the bias voltage applied to the control terminal 5is increased, the current amount flowing through the transistor 8increases, so that the current amount flowing through the diode 2correspondingly increases, with the result that the impedance Z_(D) ofthe diode 2 lowers in accordance with the equation (2), so that theinput signal is bypassed to the power supply terminal 11. As a result,the gain of the whole circuit lowers

To the contrary, if the bias voltage applied to the control terminal 5is decreased, the transistor 8 approaches the off condition, so that thecurrent amount flowing through the diode 2 correspondingly decreases, sothat the impedance Z_(D) of the diode 2 becomes large, with the resultthat the proportion of the input signal actually applied to theamplifying circuit 1 is increased, so that the gain of the whole circuitelevates. Furthermore, when the bias voltage applied to the controlterminal is made smaller than an on-voltage between the base and theemitter of the transistor 8, the current no longer flows through thediode 2, so that the impedance becomes very large, and therefore, all ofthe input signal is supplied to the amplifying circuit 1. Accordingly,the gain increases. At this time, the S/N ratio of the whole circuitbecomes large so that the noise factor NF becomes small.

The gain control circuit shown in FIG. 5 and the gain control circuitshown in FIG. 9 operate similarly to each other. In the gain controlcircuit shown in FIG. 5, the cathode of the diode 2 is connecteddirectly to the collector of the transistor 8, and on the other hand, inthe gain control circuit shown in FIG. 9, the cathode of the diode 2 isconnected through the resistor 6 to the collector of the transistor 8.In the gain control circuit shown in FIG. 5, the anode of the diode 2 isconnected through the resistor 6 to the power supply terminal 11connected to the constant voltage power supply, and on the other hand,in the gain control circuit shown in FIG. 9, the anode of the diode 2 isconnected directly to the power supply terminal 11 connected to theconstant voltage power supply. However, in both the gain controlcircuits shown in FIGS. 5 and 9, the emitter of the transistor 8 isconnected directly to the ground, and the control terminal 5 connectedto the base of the transistor 8 is connected to the ground in an ACmode. Furthermore, the collector of the transistor 8 is connectedthrough the capacitor 9 is connected to the input of the amplifyingcircuit 1. By controlling the bias voltage at the control terminal 5 tochange the current amount flowing through the diode 2, the impedance ofthe diode 2 changes, with the result that the shunt amount of the inputsignal supplied to the input terminal 3 is controlled, and therefore,the amplification gain of the amplifying circuit 1 changes.

In these gain control circuits, therefore, if the bias voltage at thecontrol terminal 5 is decreased, it is possible to adjust the variablegain of the whole circuit so as to elevate the gain and to lower thenoise factor NF while maintaining the SIN ratio of the whole circuit ata high level.

The gain control circuit shown in FIG. 6 is a variation of the gaincontrol circuit shown in FIG. 1 obtained by changing the position of theDC blocking capacitor 9. Therefore, the operation of the gain controlcircuit shown in FIG. 6 is fundamentally the same as that of the gaincontrol circuit shown in FIG. 1.

The gain control circuit shown in FIG. 7 operates similarly to the gaincontrol circuit shown in FIG. 3. In both the gain control circuits shownin FIGS. 3 and 7, the anode of the diode 2 is connected to the emitterof the transistor 8, and the cathode of the diode 2 is connected throughthe resistor 6 to the ground. The collector of the transistor 8 isconnected to the power supply terminal 11 connected to the constantvoltage power supply, and the control terminal 5 connected to the baseof the transistor 8 is connected to the ground in an AC mode.Furthermore, the emitter of the transistor 8 is connected through thecapacitor 9 is connected to the input of the amplifying circuit 1. Bycontrolling the bias voltage at the control terminal 5 to change thecurrent amounts flowing through the transistor 8 and through the diode2, the impedance of the transistor 8 and the diode 2 changes, with theresult that the shunt amount of the input signal supplied to the inputterminal 3 is controlled, and therefore, the amplification gain of theamplifying circuit 1 changes.

In these gain control circuits, therefore, if the bias voltage at thecontrol terminal 5 is decreased, it is possible to adjust the variablegain of the whole circuit so as to elevate the gain and to lower thenoise factor NF while maintaining the S/N ratio of the whole circuit ata high level.

In conclusion, in each of the above mentioned embodiments andmodifications of the gain control circuit, the adjustable impedancecircuit part is characterized by connecting the diode 2 which has asubstantially constant voltage drop when the diode 2 is in an oncondition, in series to either the emitter or the collector of thetransistor 8 which assumes a high impedance when the transistor 8 isoff. Therefore, when the bias voltage at the control terminal 5connected to the base of the transistor 8 is decreased to adjust thevariable gain of the whole circuit by action of the adjustable impedancecircuit part, it is possible to allow the amplifying circuit 1 in thevariable-gain amplifying circuit part having a high S/N ratio, tooperate under a low power supply voltage with a low noise factor.

Incidentally, in the above mentioned embodiments and modifications, ithas been described that the collector-emitter voltage drop of thetransistor 8 is 1.0V and the base-emitter voltage drop is 1.4V, whichare typical values in silicon bipolar transistors. However, even if thebipolar transistor 8 has different voltage drops, if the total valueobtained by adding the voltage drop of the diode 2 to thecollector-emitter voltage drop of the transistor 8 is smaller than thepower supply voltage, the circuit is operable.

In addition, it has been described that the transistor 8 is an NPNtransistor, however, it would be apparent to persons skilled in the artthat the transistor 8 can be constituted of a PNP transistor in place ofthe NPN transistor. In this case, an emitter of the PNP transistor isconnected to a position to which the collector of the NPN transistor wasconnected, and a collector of the PNP transistor is connected to aposition to which the emitter of the NPN transistor was connected. Abase of the PNP transistor is connected to a position to which the baseof the NPN transistor was connected.

The invention has thus been shown and described with reference to thespecific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the illustratedstructures but changes and modifications may be made within the scope ofthe appended claims.

What is claimed is:
 1. A variable-gain amplifier circuit comprising: anadjustable impedance circuit part including a bipolar transistor havinga base connected to a control terminal for receiving a controlled biasvoltage, one of a collector and an emitter of said bipolar transistorbeing connected to a first power supply terminal, and the other of saidcollector and said emitter of said bipolar transistor being connected toa second power supply terminal through a diode located in a forwarddirection to said second power supply terminal; and a variable-gainamplifying circuit part having an amplifying circuit having an inputcoupled to an input terminal for receiving an input signal and an outputconnected to an output terminal, wherein a connection point between saidbipolar transistor and said diode is connected through a DC blockingcapacitor to said input of said amplifying circuit, and wherein thediode is located in a forward direction when an anode of the diodeassumes a higher potential than the cathode.
 2. A variable-gainamplifier circuit claimed in claim 1 wherein a resistor is connected inseries with said diode between the other of said collector and saidemitter of said bipolar transistor and said second power supplyterminal.
 3. A variable-gain amplifier circuit comprising: an adjustableimpedance circuit part including a bipolar transistor having a baseconnected to a control terminal for receiving a controlled bias voltage,one of a collector and an emitter of said bipolar transistor beingconnected to a first power supply terminal, and the other of saidcollector and said emitter of said bipolar transistor being connected toa second power supply terminal through a diode located to said secondpower supply terminal, and a variable-gain amplifying circuit parthaving an amplifying circuit having an input coupled to an inputterminal for receiving an input signal and an output connected to anoutput terminal, the other of said collector and said emitter of saidbipolar transistor connected to said second power supply terminalthrough said diode and being connected through a DC blocking capacitorto said input of said amplifying circuit, wherein said first powersupply terminal is a positive power supply voltage terminal and saidsecond power supply terminal is connected to ground, said collector ofsaid bipolar transistor being connected to said positive power supplyvoltage terminal, and said emitter of said bipolar transistor beingconnected to said ground through said diode connected in such a mannerthat an anode of said diode is connected to said emitter of saidtransistor and a cathode of said diode is connected to said ground, andwherein said input of said amplifying circuit is connected through saidDC blocking capacitor to said input terminal, and said emitter of saidbipolar transistor is connected to a node between said DC blockingcapacitor and said input terminal, and wherein the diode is located in aforward direction when an anode of the diode assumes a higher potentialthan the cathode.
 4. A variable-gain amplifier circuit claimed in claim3 wherein a resistor is connected in series with said diode between saidemitter of said bipolar transistor and said ground.
 5. A variable-gainamplifier circuit comprising: an adjustable impedance circuit partincluding a bipolar transistor having a base connected to a controlterminal for receiving a controlled bias voltage, one of a collector andan emitter of said bipolar transistor being connected to a first powersupply terminal, and the other of said collector and said emitter ofsaid bipolar transistor being connected to a second power supplyterminal through a diode located to said second power supply terminal;and a variable-gain amplifying circuit part having an amplifying circuithaving an input coupled to an input terminal for receiving an inputsignal and an output connected to an output terminal, the other of saidcollector and said emitter of said bipolar transistor connected to saidsecond power supply terminal through said diode and being connectedthrough a DC blocking capacitor to said input of said amplifyingcircuit, wherein said first power supply terminal is connected to groundand said second power supply terminal is a positive power supply voltageterminal, said emitter of said bipolar transistor being connected tosaid ground, and said collector of said bipolar transistor beingconnected to said positive power supply voltage terminal through saiddiode connected in such a manner that an anode of said diode isconnected to said positive power supply voltage terminal and a cathodeof said diode is connected to said collector of said bipolar transistor,and wherein said input of said amplifying circuit is connected throughsaid DC blocking capacitor to said input terminal, and said collector ofsaid bipolar transistor is connected to a node between said DC blockingcapacitor and said input terminal, and wherein the diode is located in aforward direction when an anode of the diode assumes a higher potentialthan the cathode.
 6. A variable-gain amplifier circuit claimed in claim5 wherein a resistor is connected in series with said diode between saidcollector of said bipolar transistor and said positive power supplyvoltage terminal.
 7. A variable-gain amplifier circuit comprising: anadjustable impedance circuit part including a bipolar transistor havinga base connected to a control terminal for receiving a controlled biasvoltage, one of a collector and an emitter of said bipolar transistorbeing connected to a first power supply terminal, and the other of saidcollector and said emitter of said bipolar transistor being connected toa second power supply terminal through a diode located to said secondpower supply terminal; and a variable-gain amplifying circuit parthaving an amplifying circuit having an input coupled to an inputterminal for receiving an input signal and an output connected to anoutput terminal, the other of said collector and said emitter of saidbipolar transistor connected to said second power supply terminalthrough said diode and being connected through a DC blocking capacitorto said input of said amplifying circuit, wherein said first powersupply terminal is a positive power supply voltage terminal and saidsecond power supply terminal is ground, said collector of said bipolartransistor being connected to said positive power supply voltageterminal, and said emitter of said bipolar transistor being connected tosaid ground through said diode connected in such a manner that an anodeof said diode is connected to said emitter of said transistor and acathode of said diode is connected to said ground, and wherein saidinput of said amplifying circuit is connected directly to said inputterminal, and said emitter of said bipolar transistor is connectedthrough said DC blocking capacitor to said input of said amplifyingcircuit, and wherein the diode is located in a forward direction when ananode of the diode assumes a higher potential than the cathode.
 8. Avariable-gain amplifier circuit claimed in claim 7 wherein a resistor isconnected in series with said diode between said emitter of said bipolartransistor and said ground.
 9. A variable-gain amplifier circuitcomprising: an adjustable impedance circuit part including a bipolartransistor having a base connected to a control terminal for receiving acontrolled bias voltage, one of a collector and an emitter of saidbipolar transistor being connected to a first power supply terminal, andthe other of said collector and said emitter of said bipolar transistorbeing connected to a second power supply terminal through a diodelocated to said second power supply terminal; and a variable-gainamplifying circuit part having an amplifying circuit having an inputcoupled to an input terminal for receiving an input signal and an outputconnected to an output terminal, the other of said collector and saidemitter of said bipolar transistor connected to said second power supplyterminal through said diode and being connected through a DC blockingcapacitor to said input of said amplifying circuit, wherein said firstpower supply terminal is connected to ground and said second powersupply terminal is a positive power supply voltage terminal, saidemitter of said bipolar transistor being connected to said ground, andsaid collector of said bipolar transistor being connected to saidpositive power supply voltage terminal through said diode connected insuch a manner that an anode of said diode is connected to said positivepower supply voltage terminal and a cathode of said diode is connectedto said collector of said bipolar transistor, and wherein said input ofsaid amplifying circuit is connected directly to said input terminal,and said collector of said bipolar transistor is connected through saidDC blocking capacitor to said input of said amplifying circuit, whereinthe diode is located in a forward direction when an anode of the diodeassumes a higher potential than the cathode.
 10. A variable-gainamplifier circuit claimed in claim 9 wherein a resistor is connected inseries with said diode between said collector of said bipolar transistorand said positive power supply voltage terminal.
 11. A variable-gainamplifier circuit comprising: a bipolar transistor having a baseconnected to a control terminal for receiving a controlled bias voltage,one of a collector and an emitter of said bipolar transistor beingconnected to a first power supply terminal, and the other of saidcollector and said emitter of said bipolar transistor being connected toa second power supply terminal through a diode located in a forwarddirection to said second power supply terminal, an amplifying circuithaving an input coupled to an input terminal for receiving an inputsignal and an output connected to an output terminal, wherein the diodeis located in a forward direction when an anode of the diode assumes ahigher potential than the cathode, wherein a connection point betweensaid bipolar transistor and said diode is connected through a DCblocking capacitor to said input of said amplifying circuit, and whereinby controlling said controlled bias voltage applied to said base of saidtransistor, it is possible to adjust a gain of the variable-gainamplifier circuit, and at the same time, it is possible to allow saidamplifying circuit having a high S/N ratio, to operate under a low powersupply voltage with a low noise factor.
 12. A variable-gain amplifiercircuit claimed in claim 11 wherein a resistor is connected in serieswith said diode between the other of said collector and said emitter ofsaid bipolar transistor and said second power supply terminal.
 13. Avariable-gain amplifier circuit comprising: an adjustable impedancecircuit part including a bipolar transistor having a base connected to acontrol terminal for receiving a controlled bias voltage, one of acollector and an emitter of said bipolar transistor being connected to afirst power supply terminal, and the other of said collector and saidemitter of said bipolar transistor being connected to a second powersupply terminal through a diode located in a forward direction to saidsecond power supply terminal, a variable-gain amplifying circuit parthaving an amplifying circuit having an input coupled to an inputterminal for receiving an input signal and an output connected to anoutput terminal, the other of said collector and said emitter of saidbipolar transistor connected to said second power supply terminalthrough said diode and being connected through a DC blocking capacitorto said input of said amplifying circuit, wherein the diode is locatedin a forward direction when an anode of the diode assumes a higherpotential than the cathode, wherein by controlling said controlled biasvoltage applied to said base of said transistor, it is possible toadjust a gain of the variable-gain amplifier circuit, and at the sametime, it is possible to allow said amplifying circuit having a high S/Nratio, to operate under a low power supply voltage with a low noisefactor, wherein said first power supply terminal is a positive powersupply voltage terminal and said second power supply terminal isconnected to ground, said collector of said bipolar transistor beingconnected to said positive power supply voltage terminal, and saidemitter of said bipolar transistor being connected to said groundthrough said diode connected in such a manner that an anode of saiddiode is connected to said emitter of said transistor and a cathode ofsaid diode is connected to said ground, and wherein said input of saidamplifying circuit is connected through said DC blocking capacitor tosaid input terminal, and said emitter of said bipolar transistor isconnected to a node between said DC blocking capacitor and said inputterminal.
 14. A variable-gain amplifier circuit claimed in claim 13wherein a resistor is connected in series with said diode between saidemitter of said bipolar transistor and said ground.
 15. A variable-gainamplifier circuit comprising: a bipolar transistor having a baseconnected to a control terminal for receiving a controlled bias voltage,one of a collector and an emitter of said bipolar transistor beingconnected to a first power supply terminal, and the other of saidcollector and said emitter of said bipolar transistor being connected toa second power supply terminal through a diode located in a forwarddirection to said second power supply terminal, an amplifying circuithaving an input coupled to an input terminal for receiving an inputsignal and an output connected to an output terminal, the other of saidcollector and said emitter of said bipolar transistor connected to saidsecond power supply terminal through said diode and being connectedthrough a DC blocking capacitor to said input of said amplifyingcircuit, wherein the diode is located in a forward direction when ananode of the diode assumes a higher potential than the cathode, whereinby controlling said controlled bias voltage applied to said base of saidtransistor, it is possible to adjust a gain of the variable-gainamplifier circuit, and at the same time, it is possible to allow saidamplifying circuit having a high S/N ratio, to operate under a low powersupply voltage with a low noise factor, wherein said first power supplyterminal is connected to ground and said second power supply terminal isa positive power supply voltage terminal, said emitter of said bipolartransistor being connected to said ground, and said collector of saidbipolar transistor being connected to said positive power supply voltageterminal through said diode connected in such a manner that an anode ofsaid diode is connected to said positive power supply voltage terminaland a cathode of said diode is connected to said collector of saidbipolar transistor, and wherein said input of said amplifying circuit isconnected through said DC blocking capacitor to said input terminal, andsaid collector of said bipolar transistor is connected to a node betweensaid DC blocking capacitor and said input terminal.
 16. A variable-gainamplifier circuit claimed in claim 15 wherein a resistor is connected inseries with said diode between said collector of said bipolar transistorand said positive power supply voltage terminal.
 17. A variable-gainamplifier circuit comprising: a bipolar transistor having a baseconnected to a control terminal for receiving a controlled bias one of acollector and an emitter of said bipolar transistor being connected to afirst power supply terminal, and the other of said collector and saidemitter of said bipolar transistor being connected to a second powersupply terminal through a diode located in a forward direction to saidsecond power supply terminal, an amplifying circuit having an inputcoupled to an input terminal for receiving an input signal and an outputconnected to an output terminal, the other of said collector and saidemitter of said bipolar transistor connected to said second power supplyterminal through said diode and being connected through a DC blockingcapacitor to said input of said amplifying circuit, wherein the diode islocated in a forward direction when an anode of the diode assumes ahigher potential than the cathode, wherein by controlling saidcontrolled bias voltage applied to said base of said transistor, it ispossible to adjust a gain of the variable-gain amplifier circuit, and atthe same time, it is possible to allow said amplifying circuit having ahigh S/N ratio, to operate under a low power supply voltage with a lownoise factor, wherein said first power supply terminal is a positivepower supply voltage terminal and said second power supply terminal isground, said collector of said bipolar transistor being connected tosaid positive power supply voltage terminal, and said emitter of saidbipolar transistor being connected to said ground through said diodeconnected in such a manner that an anode of said diode is connected tosaid emitter of said transistor and a cathode of said diode is connectedto said ground, and wherein said input of said amplifying circuit isconnected directly to said input terminal, and said emitter of saidbipolar transistor is connected through said DC blocking capacitor tosaid input of said amplifying circuit.
 18. A variable-gain amplifiercircuit claimed in claim 17 wherein a resistor is connected in serieswith said diode between said emitter of said bipolar transistor and saidground.
 19. A variable-gain amplifier circuit comprising: a bipolartransistor having a base connected to a control terminal for receiving acontrolled bias voltage, one of a collector and an emitter of saidbipolar transistor being connected to a first power supply terminal, andthe other of said collector and said emitter of said bipolar transistorbeing connected to a second power supply terminal through a diodelocated in a forward direction to said second power supply terminal, anamplifying circuit having an input coupled to an input terminal forreceiving an input signal and an output connected to an output terminal,the other of said collector and said emitter of said bipolar transistorconnected to said second power supply terminal through said diode andbeing connected through a DC blocking capacitor to said input of saidamplifying circuit, wherein the diode is located in a forward directionwhen an anode of the diode assumes a higher potential than the cathode,wherein by controlling said controlled bias voltage applied to said baseof said transistor, it is possible to adjust a gain of the variable-gainamplifier circuit, and at the same time, it is possible to allow saidamplifying circuit having a high S/N ratio, to operate under a low powersupply voltage with a low noise factor, wherein said first power supplyterminal is connected to ground and said second power supply terminal isa positive power supply voltage terminal, said emitter of said bipolartransistor being connected to said ground, and said collector of saidbipolar transistor being connected to said positive power supply voltageterminal through said diode connected in such a manner that an anode ofsaid diode is connected to said positive power supply voltage terminaland a cathode of said diode is connected to said collector of saidbipolar transistor, and wherein said input of said amplifying circuit isconnected directly to said input terminal, and said collector of saidbipolar transistor is connected through said DC blocking capacitor tosaid input of said amplifying circuit.
 20. A variable-gain amplifiercircuit claimed in claim 19 wherein a resistor is connected in serieswith said diode between said collector of said bipolar transistor andsaid positive power supply voltage terminal.
 21. A variable-gainamplifier circuit comprising: an adjustable impedance circuit partincluding a bipolar transistor having a base connected to a controlterminal for receiving a controlled bias voltage, an emitter of saidbipolar transistor being connected to ground, and a collector of saidbipolar transistor being connected to a cathode of a diode having ananode connected to a positive power supply terminal; and a variable-gainamplifying circuit part having an amplifying circuit having an inputcoupled through a DC blocking capacitor to an input terminal forreceiving an input signal and an output connected to an output terminal,a connection point between said collector of said bipolar transistor andsaid cathode of said diode being connected to a connection node betweensaid DC blocking capacitor and said input terminal.
 22. A variable-gainamplifier circuit claimed in claim 21 wherein a resistor is connected inseries with said diode between said positive power supply terminal andsaid connection node between said collector of said bipolar transistor,said DC blocking capacitor and said input terminal.